| Bacteria grows easily on fresh meat
Meat and meat products are particularly susceptible to bacterial growth due to their high water activity and nutrient content. When cut, meat surfaces exposed to ambient air provide excellent breeding grounds for most bacteria. Minced meat is even more at risk due to the large surface area exposed. For this reason, rigorous hygiene in all processing and prepackaging steps is vital. All tools and equipment must be kept clean to minimize the risk of contact with contaminating microorganisms.
Bacteria count over time for meat stored in air and in a modified atmosphere at the same temperature
Carbon dioxide to the rescue
Aerobic bacteria from the pseudomonas genus present the biggest threat to fresh meat. Generally speaking, carbon dioxide (CO2) has a strong inhibiting effect on the growth of this and other bacteria.
Red meat requires oxygen
Oxidation of the red pigment in red meats such as beef causes color changes. To preserve the red color of the meat, the atmosphere for fresh meat normally contains high concentrations of oxygen (60% - 80%). This ensures high oxygen levels in the meat's myoglobin.
Highly pigmented meats (such as beef) thus require higher oxygen concentrations than meats with low levels of pigmentation (such as pork).
With the right mixtures, the practical shelf-life of consumer-packaged meats can be extended from between 2 and 4 days to between 5 and 8 days at +4°C. If master packs are used in distribution, high CO2 levels can be used to increase shelf-life.
Poultry
Poultry is very susceptible to bacterial spoilage, evaporation loss, off-odor, discoloration and biochemical deterioration. The sterile poultry tissue becomes contaminated during the evisceration process. The practical shelf-life of gas-packed poultry is somewhere between 16 and 21 days. The head space volume should be nearly as large as the product volume. In contrast to red meats, poultry does not undergo irreversible discoloration of the meat’s surface in the presence of O2.
The spoilage of raw poultry is mainly caused by microbial growth, fuelled by the pseudomonas and achromobacter genera in particular. These aerobic bacteria are effectively inhibited by CO2 in MAP. Levels of CO2 in excess of 20% are required to significantly extend the shelf-life of poultry.
To avoid problems associated with collapsed packaging and excessive dripping with raw poultry, the gas/product ratio should be increased if higher levels of CO2 are used. Where package collapse is not an issue (e.g. bulk or master bags), 100% CO2 is recommended. In both retail and bulk modified atmosphere packs, N2 is used as an inert filler gas.
A = 100 % CO2 / 1 °C
B = 10 - 30 % CO2 rest N2 / 1 °C
C = Vacuum / 4 - 6 °C
D = 100 % CO2 / 4 - 6 °C
E = Vacuum / 4 - 6 °C
F = 10 - 30 % CO2 rest N2 / 4 - 6 °C
G = Air / 1 °C
H = Air / 4 - 6 °C
Meat products have different microflora
Deterioration of meat products is most commonly caused by microbial spoilage. As a result of processing (e.g. marinating, drying, smoking, fermenting, curing and cooking), the microflora in processed meat products differs from that of raw meat.
As a result, the spoilage mechanisms vary. This affects the gas composition most advisable for the package. The concentration of CO2 is usually low (20 –50%) to avoid the meat going sour.
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